At a 2015 Q&A at BAM Cinematek, John Korty revealed why there was a version of the film with strong language. The film was always intended to be appropriate for young children, and Korty's final cut was almost identical to the 1991 home video release. After answer prints had been made based off his final cut, Korty went to New York to begin pre-production on another project while test screenings of "Twice Upon a Time" were taking place in California. Bill Couturié one of the film's producers and co-writers, noted that college-aged audience members frequently walked out during the first ten minutes. After nearly the entire audience at a screening at USC walked out, Couturié reassembled some of the voice actors to record more profane versions of the film's dialogue in the hopes that it would keep teens and adults interested enough to watch the film to completion. John Korty was kept completely out of the loop on these changes, and didn't find out until he was watching the film at its premiere. Korty was furious, even more so when he found out release prints were made using this more profane version. Theater programmers didn't know these changes had been made either, and one of the few theaters in the country to show the film on its original release ran it on a double bill with the G-rated Brisby et le secret de NIMH (1982). When outraged parents notified theater management, Twice was pulled from further showings.
David Fincher was only 17 when he did the film's visual effects. He's responsible for some of the opening shots of the Murkworks.
The credits sequence features a photo of voice actor Lorenzo Music, who was notorious for not allowing pictures of himself to be seen.
Henry Selick future director of L'Étrange Noël de monsieur Jack (1993), James et la Pêche géante (1996), Monkeybone (2001), Coraline (2009) and Wendell et Wild (2022) directed the sequence where Ralph and Mumford experience a nightmare.